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Effects of auxiliary complex-forming agents on the rate of metallochromic indicator colour change-IV1
669
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Summary-Polyoxyethylated non-ionic surfactants such as Tween 20, Tween 40, Nonidet P40 and Nonex 501 have been supposed to be associated with cationic characteristics. Studies on the effect of these surfactants on the electrocapillary curves of the anionic surfactants Aerosol IB, Manaxol OT and sodium lauryl sulphate (SLS), show that the electrocapillary maxima shift towards positive potentials. The order of adsorption of the anionic surfactants is SLS ) Manaxol OT j Aerosol IB while OT j SLS which confirms association of the shift in maxima is m the order Aerosol IB - Manaxol cationic characteristics with the micelles of these non-ionic surfactants. The magnitude of the shift m electrocapillary maxima is Nonex 501 ) Nonidet P40) Tween 20 j Tween 40 which may be the order of magnitude of the positive charge carried by these non-ionic surfactants.
Tulanro. Vol 23.pp 669411 PergdmonPress, 1976Prmted m Great Bntam
EFFECTS OF AUXILIARY COMPLEX-FORMING AGENTS ON THE RATE OF METALLOCHROMIC INDICATOR COLOUR CHANGE-IV* MECHANISM OF THE COLOUR CHANGE OF XYLENOL ORANGE IN COPPER(IItEDTA TITRATIONS HIROKO
WADA,TOMOSUKE
Laboratory
of Analytical Gokiso-cho,
ISHIZUKI and
Chemistry, Showa-ku,
Nagoya Nagoya,
GENKICHI
NAKAGAWA
Institute of Technology, 466, Japan
(Recerved 12 March 1976. Accepted 18 March 1976)
In the copper(IItEDTA titration with Xylenol Orange (X0) as indicator, hexamine slows down the rate of colour change of X0. 1,2 In the work described here, the rate of the substitution reaction of the copper(IIkX0 chelate with EDTA was determined in MES buffer [2-(N-morpho1mo)ethanesulphonic acid] and in hexamine buffer, and the mechanism of the disturbing effect of hexamine on the colour change of the indicator discussed. EXPERIMENTAL
and purified in a manner similar to that in the literature.3 The free acid form (H,XO) was dissolved in water, and the solution stored in a refrigerator. Dissociation constants of X0 determined by spectrophotometry and pH-titration were in good agreement with the values given by Murakami et ~1.~ A copper(H) solution was prepared from the reagentgrade nitrate. Reagent-grade hexamme dried over phosphorus pentoxtde was used without further purification. The purity was established as 99.0% by means of pH-titration with sodium hydroxide in the presence of excess of hydrochloric acid. Other reagents and apparatus employed were the same as those reported previously.4 All experiments were carried out at 25 f I” and at ionic strength of 0.1 (KNO,). X0
with 0.198M sodium hydroxide shows the formation of Cu,HXOat pH 2, and further release of one proton, which corresponds to the sixth proton of H,XO, takes place in the pH-range from 3.5 to 5.5. In thts pH-range the absorption maximum shifts from 440 to 574nm, and an isosbestic point occurs at 487 nm. From these results the following equilibrium exists: Cu,XO*+ H+ $ Cu2HXO-. The equilibrium constant, K&x0 = [Cu,HXO-]/[Cn,XO*-][H’], was evaluated as lo4 ss by spectrophotometry and pH-titration.
was synthesized
06-
04s 5 f :: 9
RESULTS AND DISCUSSION
The composrtlon of copper(lltX0
chelates
From the results of the contmuous variation method, the molar-ratio method by spectrophotometry and the potentiometric titration wtth use of a copper(H) ion-selective electrode, the ratio of copper to X0 was essentially 2: 1 in MES buffer. The pH-titration of a solution 3.76 x 10e3M in Cu and 0.934 x 10m3M in H,XO * Part III: Talanta, 1976, 23, 155.
500
600
550 Wavelength,
nm
Fig. 1. Spectra of Cu-X0 chelates. Cc. 1.6 x 10m4!vf, Cxo 1.1 x lO-‘M. 1, X0 blank; 2, Cu,XO*-; 3, Cu,XOL*-, Chex 2.0 x lo-‘M, pH 6.0.
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At pH 6 in the presence of hexamine the absorption spectrum of Cu,XO*shifts to longer wavelengths (&,.,, = 578 nm) and the molar absorptivity increases with increase in the hexamine concentration (Fig. 1). This indicates the formation of a mixed-ligand complex of the copper-X0 chelate with hexamine. The formation constant of the mixed-ligand complex is defined as: KnL CUIXOL
CC%XOL,z -1 =
~~u,xoz-][L]”
(1)
of the 2: 1 chelate (Cu,XO’-) may support the mechanism proposed in equations (6) and (7). In the presence of hexamine. A solution contaimng X0, Cu and hexamine (2.83 x 10-4-2.19 x lo-‘M) was mixed with a solution containing EDTA, and the absorbance at 578 nm was measured in the pH range from 5.77 to 6.24 (MES buffer) in the same way as for the MES buffer system. The substitution reaction is written as: Cu,XOL + EDTA +2Cu(EDTA)
where L represents free hexamine. The absorbances of Cu,XOLiat 578nm were measured for varying hexamine concentration from 10m3 to 10-‘M at pH 6.0&6.30. A plot of log[Cu,XOLi-]/[Cu,XO’-] us. log[L] yielded a straight line with slope of 1. Therefore, one molecule of hexamine co-ordinates with Cu,XO*-. The value of was evaluated as lo2 14, The dissociation constant ?%? was taken as 10e4 98. The rate of substitution of Cu,XO with EDTA In MES buffer. A solution 2.10-5.25 x lo-‘M in X0, 3.87-7.74 x lo- ‘M in Cu and 0.02M in MES-NaOH buffer (pH 4.82-6.00) and a solution 3.5614.25 x 10m4M in EDTA and 0.02M in MES-NaOH buffer (pH 4.82-6.00) were mixed, and the absorbance at 574 nm was measured as a function of the reaction time by the stopped-flow method. Under these experImenta conditions the substitution reaction of the Cu,XO*- chelate with EDTA proceeds to completion. CuzXO + 2EDTA = 2Cu(EDTA) + X0
(3) v) 1s the conditional rate-constant involving where kO(H,R,M the concentrations of hydrogen Ion, X0, Cu and EDTA. From equation (3) we obtain (4)
where A,,, A, and A,, represent the absorbances of the reaction system at t = 0, t and co, respectively. The plots of log(A, - A,) vs. t yielded straight lines up to 90% of the total reaction, and the conditional rate-constants k OcH,R,M,Yj were obtained from the slopes of the straight lines. The data in Table 1 indicate that the values of k, are proportional to the concentration of EDTA but are independent of hydrogen Ion, X0 and Cu concentrations. Hence,
_
!!h??o’:~= k,[y’][CUJ02-]
Cu*XOZ~ + Y’ $ YCu(XO)Cu 2 CuHX03-
or
d[Cu,XOL* -1 - ---____dt = k,~~.~,~,~.~,CCu,XOL2-1
(9)
The results are shown in Table 2. No dependence of kOcH,R,M,Y,Lj on the concentrations of X0, Cu and hydrogen ion was observed, but there was a linear relation to EDTA concentration. With increasing concentration of hexamine the rate of the substitution reaction decreased. In the hexamine concentration range from 3.00 x lo-* to 2.19 x 10- 'M, where the mixed-ligand complex Cu,XOL’forms quantitatively, k,, was proportional to the reciprocal of the concentration of hexamine. Thus, the rate-law may be rewritten as d[Cu,XOL’-] dt
cu,XOLZ-
CuH,XO’-
The release of the first copper ion from the Cu,XO’- chelate may be the rate-determining step (r.d.s.). From the results of potentlometry with a copper(I1) ion-selective electrode the resulting 1: 1 chelate is in the form of CuHX03- or CuH,X02-, and the protonation of the 1: 1 chelate may weaken the Cu-X0 bonds. The high stability
+ L
(11)
= 7.44 x lO*l.mole-‘.sec-’ This value is in good agreement with that obtained in the absence of hexamine. This fact supports the reaction mechanism described above. Table
1.
First-order
conditxonal rate constants
RM
kocH
y)
,
25’C,
II
-
0.1
3.56
5.34
3.15 4.20 5.25 2.10
3.15 4.20 2.10
(6) (7)
= cu,xoz-
k, = k2K&xOL = 5.39 x IO* l4
CuH2XOZ- + Y’% CuY’- + H,X03-
(10)
CL’1
The substitution mechanism of Cu,XO*- with EDTA may be the same as that in the absence of hexamine. From equations (I), (5) and (10):
7.12
or
_ k ~~‘~?~?!&!!!z - ]
The value of k, was evaluated as 5.39 set-‘. Therefore, the dissociation of hexamine from Cu,XOL’- takes place first:
(5)
where [Y’] is the total concentration of EDTA ([HzYz-] + [HY3-1) not combined with copper ion, and the rate-constant k, was evaluated as 7.84 x lo* 1. mole. set-‘. The following reaction mechanism may be proposed. CuY’- + CuHX03-
(8)
(2)
The rate-law is expressed by
log@, - A,) = - kO@*R&J m2,jo3 t + log(A, - A,)
+ X0 + L
The rate-law is
10.7
14.3
3.15 4.20 5.25 7.10
4.87 5.59 5.68 5.80 5.85 6.00 5.39
4.82 5.51 5.80 5.85 6.00 5.39 5.68 5.85 5.68
2.90 2.65 2.68 2.46 2.41 2.30 2.74 2.78 2.43 4.32 4.03 4.29 4.22 4.34 4.09 3.83 4.09 4.17 4.09 5.21 5.60 5.10 5.30 5.47 5.28 5.89 8.64 8.24 7.90 8.00 8.46 8.24 8.59 10.3 10.7 11.7 10.6
I
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671
Fig. 2. Plot of logk VS.log[L]. Solid line is the theoretical curve.
At lower concentrations of hexamine Cu,X02and Cu,XOL’complexes exist. Thus, the rate law can be expressed as
in Fig. 2. The values of K&xOL, k, and k2 obtained by a curve-fitting method were 10’ Is, 7.76 x 10’ 1. mole-‘. set-’ and 5.13 set-‘, respectively. These values agree well with those obtained above. The curve shows that the presence of more than lo-*M hexamine markedly slows down the rate of colour change of the indicator.
REFERENCES
=(
(12)
where [(Cu,XO)‘] represents the total concentration of CuXO chelates. The plot of log k vs. log[L] is shown
1. R. Ptibil, Tulanta, 1959, 3, 91. 2. G. Nakagawa and H. Wada, ibid., 1973, 20, 829. 3. M. Murakami, T. Yoshino and S. Harasawa, ibid., 1967, 14, 1293. 4. G. Nakagawa and H. Wada, ibid., 1975, 22, 563.
Summary-The rate of ligand substitution of copper(IIbXyleno1 Orange (X0) with ElYTA (Y) has been determined spectrophotometrically over the pH range 4.8-6.0 at p = 0.1 (KN03) and at 25”. In 2-(N-morpholino)ethane sulphonic acid buffer, copper forms a 2:l chelate (Cu2XOz-) with X0, and the rate-law is expressed as -d[Cu,XO’-]/dt = 102~89[Cu2X02-]cY,]. The release of the first copper ion from CuaXO’- is the rate-determining step. The resulting CuHX03- or CuHzXOz- may undergo fast substitution with EDTA. In the presence of hexamine, the copper(IIbX0 chelate forms a mixed-ligand complex with hexamine (L). The formation constant K&xOL = [Cu,XOL2-]/ [cu2xo*-] [L] = 102 i4 (p = 0.1, 25”). At 3 x lo-‘-2 x lo-‘M hexamine the rate-law is expressed as -d[Cu,XOL2-]/dt = 5.39[Cu,XOL2-] cy’]/IJ’]. The dissociation of hexamine from Cu,XOL’has to precede the substitution reaction of Cu2X02- with EDTA. Hence, hexamine at higher concentrations than 10e3M slows down the rate of colour change of X0 in the copper-EDTA titration.
SHORT COMMUNICATIONS
Summary-Polyoxyethylated non-ionic surfactants such as Tween 20, Tween 40, Nonidet P40 and Nonex 501 have been supposed to be associated with cationic characteristics. Studies on the effect of these surfactants on the electrocapillary curves of the anionic surfactants Aerosol IB, Manaxol OT and sodium lauryl sulphate (SLS), show that the electrocapillary maxima shift towards positive potentials. The order of adsorption of the anionic surfactants is SLS ) Manaxol OT j Aerosol IB while OT j SLS which confirms association of the shift in maxima is m the order Aerosol IB - Manaxol cationic characteristics with the micelles of these non-ionic surfactants. The magnitude of the shift m electrocapillary maxima is Nonex 501 ) Nonidet P40) Tween 20 j Tween 40 which may be the order of magnitude of the positive charge carried by these non-ionic surfactants.
Tulanro. Vol 23.pp 669411 PergdmonPress, 1976Prmted m Great Bntam
EFFECTS OF AUXILIARY COMPLEX-FORMING AGENTS ON THE RATE OF METALLOCHROMIC INDICATOR COLOUR CHANGE-IV* MECHANISM OF THE COLOUR CHANGE OF XYLENOL ORANGE IN COPPER(IItEDTA TITRATIONS HIROKO
WADA,TOMOSUKE
Laboratory
of Analytical Gokiso-cho,
ISHIZUKI and
Chemistry, Showa-ku,
Nagoya Nagoya,
GENKICHI
NAKAGAWA
Institute of Technology, 466, Japan
(Recerved 12 March 1976. Accepted 18 March 1976)
In the copper(IItEDTA titration with Xylenol Orange (X0) as indicator, hexamine slows down the rate of colour change of X0. 1,2 In the work described here, the rate of the substitution reaction of the copper(IIkX0 chelate with EDTA was determined in MES buffer [2-(N-morpho1mo)ethanesulphonic acid] and in hexamine buffer, and the mechanism of the disturbing effect of hexamine on the colour change of the indicator discussed. EXPERIMENTAL
and purified in a manner similar to that in the literature.3 The free acid form (H,XO) was dissolved in water, and the solution stored in a refrigerator. Dissociation constants of X0 determined by spectrophotometry and pH-titration were in good agreement with the values given by Murakami et ~1.~ A copper(H) solution was prepared from the reagentgrade nitrate. Reagent-grade hexamme dried over phosphorus pentoxtde was used without further purification. The purity was established as 99.0% by means of pH-titration with sodium hydroxide in the presence of excess of hydrochloric acid. Other reagents and apparatus employed were the same as those reported previously.4 All experiments were carried out at 25 f I” and at ionic strength of 0.1 (KNO,). X0
with 0.198M sodium hydroxide shows the formation of Cu,HXOat pH 2, and further release of one proton, which corresponds to the sixth proton of H,XO, takes place in the pH-range from 3.5 to 5.5. In thts pH-range the absorption maximum shifts from 440 to 574nm, and an isosbestic point occurs at 487 nm. From these results the following equilibrium exists: Cu,XO*+ H+ $ Cu2HXO-. The equilibrium constant, K&x0 = [Cu,HXO-]/[Cn,XO*-][H’], was evaluated as lo4 ss by spectrophotometry and pH-titration.
was synthesized
06-
04s 5 f :: 9
RESULTS AND DISCUSSION
The composrtlon of copper(lltX0
chelates
From the results of the contmuous variation method, the molar-ratio method by spectrophotometry and the potentiometric titration wtth use of a copper(H) ion-selective electrode, the ratio of copper to X0 was essentially 2: 1 in MES buffer. The pH-titration of a solution 3.76 x 10e3M in Cu and 0.934 x 10m3M in H,XO * Part III: Talanta, 1976, 23, 155.
500
600
550 Wavelength,
nm
Fig. 1. Spectra of Cu-X0 chelates. Cc. 1.6 x 10m4!vf, Cxo 1.1 x lO-‘M. 1, X0 blank; 2, Cu,XO*-; 3, Cu,XOL*-, Chex 2.0 x lo-‘M, pH 6.0.
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At pH 6 in the presence of hexamine the absorption spectrum of Cu,XO*shifts to longer wavelengths (&,.,, = 578 nm) and the molar absorptivity increases with increase in the hexamine concentration (Fig. 1). This indicates the formation of a mixed-ligand complex of the copper-X0 chelate with hexamine. The formation constant of the mixed-ligand complex is defined as: KnL CUIXOL
CC%XOL,z -1 =
~~u,xoz-][L]”
(1)
of the 2: 1 chelate (Cu,XO’-) may support the mechanism proposed in equations (6) and (7). In the presence of hexamine. A solution contaimng X0, Cu and hexamine (2.83 x 10-4-2.19 x lo-‘M) was mixed with a solution containing EDTA, and the absorbance at 578 nm was measured in the pH range from 5.77 to 6.24 (MES buffer) in the same way as for the MES buffer system. The substitution reaction is written as: Cu,XOL + EDTA +2Cu(EDTA)
where L represents free hexamine. The absorbances of Cu,XOLiat 578nm were measured for varying hexamine concentration from 10m3 to 10-‘M at pH 6.0&6.30. A plot of log[Cu,XOLi-]/[Cu,XO’-] us. log[L] yielded a straight line with slope of 1. Therefore, one molecule of hexamine co-ordinates with Cu,XO*-. The value of was evaluated as lo2 14, The dissociation constant ?%? was taken as 10e4 98. The rate of substitution of Cu,XO with EDTA In MES buffer. A solution 2.10-5.25 x lo-‘M in X0, 3.87-7.74 x lo- ‘M in Cu and 0.02M in MES-NaOH buffer (pH 4.82-6.00) and a solution 3.5614.25 x 10m4M in EDTA and 0.02M in MES-NaOH buffer (pH 4.82-6.00) were mixed, and the absorbance at 574 nm was measured as a function of the reaction time by the stopped-flow method. Under these experImenta conditions the substitution reaction of the Cu,XO*- chelate with EDTA proceeds to completion. CuzXO + 2EDTA = 2Cu(EDTA) + X0
(3) v) 1s the conditional rate-constant involving where kO(H,R,M the concentrations of hydrogen Ion, X0, Cu and EDTA. From equation (3) we obtain (4)
where A,,, A, and A,, represent the absorbances of the reaction system at t = 0, t and co, respectively. The plots of log(A, - A,) vs. t yielded straight lines up to 90% of the total reaction, and the conditional rate-constants k OcH,R,M,Yj were obtained from the slopes of the straight lines. The data in Table 1 indicate that the values of k, are proportional to the concentration of EDTA but are independent of hydrogen Ion, X0 and Cu concentrations. Hence,
_
!!h??o’:~= k,[y’][CUJ02-]
Cu*XOZ~ + Y’ $ YCu(XO)Cu 2 CuHX03-
or
d[Cu,XOL* -1 - ---____dt = k,~~.~,~,~.~,CCu,XOL2-1
(9)
The results are shown in Table 2. No dependence of kOcH,R,M,Y,Lj on the concentrations of X0, Cu and hydrogen ion was observed, but there was a linear relation to EDTA concentration. With increasing concentration of hexamine the rate of the substitution reaction decreased. In the hexamine concentration range from 3.00 x lo-* to 2.19 x 10- 'M, where the mixed-ligand complex Cu,XOL’forms quantitatively, k,, was proportional to the reciprocal of the concentration of hexamine. Thus, the rate-law may be rewritten as d[Cu,XOL’-] dt
cu,XOLZ-
CuH,XO’-
The release of the first copper ion from the Cu,XO’- chelate may be the rate-determining step (r.d.s.). From the results of potentlometry with a copper(I1) ion-selective electrode the resulting 1: 1 chelate is in the form of CuHX03- or CuH,X02-, and the protonation of the 1: 1 chelate may weaken the Cu-X0 bonds. The high stability
+ L
(11)
= 7.44 x lO*l.mole-‘.sec-’ This value is in good agreement with that obtained in the absence of hexamine. This fact supports the reaction mechanism described above. Table
1.
First-order
conditxonal rate constants
RM
kocH
y)
,
25’C,
II
-
0.1
3.56
5.34
3.15 4.20 5.25 2.10
3.15 4.20 2.10
(6) (7)
= cu,xoz-
k, = k2K&xOL = 5.39 x IO* l4
CuH2XOZ- + Y’% CuY’- + H,X03-
(10)
CL’1
The substitution mechanism of Cu,XO*- with EDTA may be the same as that in the absence of hexamine. From equations (I), (5) and (10):
7.12
or
_ k ~~‘~?~?!&!!!z - ]
The value of k, was evaluated as 5.39 set-‘. Therefore, the dissociation of hexamine from Cu,XOL’- takes place first:
(5)
where [Y’] is the total concentration of EDTA ([HzYz-] + [HY3-1) not combined with copper ion, and the rate-constant k, was evaluated as 7.84 x lo* 1. mole. set-‘. The following reaction mechanism may be proposed. CuY’- + CuHX03-
(8)
(2)
The rate-law is expressed by
log@, - A,) = - kO@*R&J m2,jo3 t + log(A, - A,)
+ X0 + L
The rate-law is
10.7
14.3
3.15 4.20 5.25 7.10
4.87 5.59 5.68 5.80 5.85 6.00 5.39
4.82 5.51 5.80 5.85 6.00 5.39 5.68 5.85 5.68
2.90 2.65 2.68 2.46 2.41 2.30 2.74 2.78 2.43 4.32 4.03 4.29 4.22 4.34 4.09 3.83 4.09 4.17 4.09 5.21 5.60 5.10 5.30 5.47 5.28 5.89 8.64 8.24 7.90 8.00 8.46 8.24 8.59 10.3 10.7 11.7 10.6
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Fig. 2. Plot of logk VS.log[L]. Solid line is the theoretical curve.
At lower concentrations of hexamine Cu,X02and Cu,XOL’complexes exist. Thus, the rate law can be expressed as
in Fig. 2. The values of K&xOL, k, and k2 obtained by a curve-fitting method were 10’ Is, 7.76 x 10’ 1. mole-‘. set-’ and 5.13 set-‘, respectively. These values agree well with those obtained above. The curve shows that the presence of more than lo-*M hexamine markedly slows down the rate of colour change of the indicator.
REFERENCES
=(
(12)
where [(Cu,XO)‘] represents the total concentration of CuXO chelates. The plot of log k vs. log[L] is shown
1. R. Ptibil, Tulanta, 1959, 3, 91. 2. G. Nakagawa and H. Wada, ibid., 1973, 20, 829. 3. M. Murakami, T. Yoshino and S. Harasawa, ibid., 1967, 14, 1293. 4. G. Nakagawa and H. Wada, ibid., 1975, 22, 563.
Summary-The rate of ligand substitution of copper(IIbXyleno1 Orange (X0) with ElYTA (Y) has been determined spectrophotometrically over the pH range 4.8-6.0 at p = 0.1 (KN03) and at 25”. In 2-(N-morpholino)ethane sulphonic acid buffer, copper forms a 2:l chelate (Cu2XOz-) with X0, and the rate-law is expressed as -d[Cu,XO’-]/dt = 102~89[Cu2X02-]cY,]. The release of the first copper ion from CuaXO’- is the rate-determining step. The resulting CuHX03- or CuHzXOz- may undergo fast substitution with EDTA. In the presence of hexamine, the copper(IIbX0 chelate forms a mixed-ligand complex with hexamine (L). The formation constant K&xOL = [Cu,XOL2-]/ [cu2xo*-] [L] = 102 i4 (p = 0.1, 25”). At 3 x lo-‘-2 x lo-‘M hexamine the rate-law is expressed as -d[Cu,XOL2-]/dt = 5.39[Cu,XOL2-] cy’]/IJ’]. The dissociation of hexamine from Cu,XOL’has to precede the substitution reaction of Cu2X02- with EDTA. Hence, hexamine at higher concentrations than 10e3M slows down the rate of colour change of X0 in the copper-EDTA titration.